DOE ECMP PI meeting, day 2 - things I learned

Very brief list of things I learned about during day 2 yesterday:

• Due to optical nonlinearities, it is possible to get broadband negative (left-handed, that is) refraction from (18 layer thick) graphene.
• In a strong perpendicular-to-plane magnetic field, you can detect (optically) evidence of 1-d subband formation and e-e interactions.
• The optical properties of graphene are very very rich.  That is, complicated.
• Doug Hofstadter was right.
• With an in-plane magnetic field, you can see physics that looks like the quantum spin Hall effect in single-layer graphene.
• Trying to tune the bandgap of GaAs down to 1 V via nitrogen doping without killing the mobility is very hard.
• Ballistic phonon pulses are a very cool way of detecting defects and interface roughness basically with sonar!
• You can measure the exchange field between a magnet and electrons in a superconductor if you can work with ultrathin films (field in plane).
• There are still some weird issues associated with electronic decoherence in the mesoscopic world - coherence times seem to saturate at the lowest temperatures in various etched semiconductor and bismuth nanowires.
• Pumping spin currents via the spin Hall effect is still cool.
• Electronic heating above the lattice temperature in graphene is more complicated than it would appear.
• Anisotropy in the electronic structure at B=0 leads to modified anisotropy in composite fermions at \( \nu = 1/2 \).
• The \( \nu = 5/2 \) quantum Hall state is surprisingly robust as mobility goes down.  That means that short-range, high-angle scattering doesn't really kill the state, which is good, and that mobility as our favorite proxy for sample quality is a poor guide in this regime, which is interesting.
• My colleague Rui-Rui Du has a really great and exciting system for looking at topological edge states and quantum spin Hall in InAs/GaSb quantum well structures available from a commercial vendor.

DOE ECMP PI meeting, day 1 - things I learned

Yesterday was an extremely dense meeting day.  Many talks, many posters.  By its nature, this meeting is far more technical than the Packard meeting, so the bullet points below are going to be more obscure to the nonexpert.  The program is here.  Among the things I learned yesterday:

• Harold Hwang continues to do very interesting physics at the interface between LaAlO3 and SrTiO3, looking at fundamental issues like the limits of charge mobility in the 2d electron gas there, and how to make delta-doped bilayers.
• Many other people are playing with oxide and pnictide MBE, making pnictide superlattices, strain-controlled pnictides, multiferroic films, etc.
• It is possible to use the elastic deformation of VO2 at the metal-insulator transition to alter the magnetic coercivity of an overlying Ni layer. 
• In strained films, it is possible to see through x-ray techniques that one can decouple the electronic transition in VO2 from the structural transition.  
• Real progress has been made recently in using engineered structures of nanomagnetic patterns to model complex systems like spin ice.
• Nd2Fe14B, the rare-earth hard magnet, can take up hydrogen into its open structure, and when it does, the lattice expands, which greatly softens the magnetic response.
• Mott insulating materials can be synthesized that exhibit quantum criticality at zero magnetic field and as-made.
• Iridates are interesting and complicated.
• Investing in developing a particular technique (in this case, NMR of unusual elements like oxygen, sodium, and arsenic) can pay big long-term dividends in terms of unique experimental insights (e.g., there are no "static loop currents" flowing in the cuprate superconducting state).

DOE experimental condensed matter physics principal investigator meeting

I am spending the next 2.5 days at the DOE's experimental CMP principal investigator meeting in the Washington DC area.  I'll try to blog a few highlights over the course of the meeting.  The basic idea is supposed to be to get all of the PIs together to talk about their latest stuff, and ideally to foster new collaborations and activities.  Judging from the people sitting around me, it looks like this will be an extremely strong meeting in terms of science.

Things I learned this week at the Packard meeting

For the 25th anniversary of the amazingly awesome David and Lucille Packard Foundation fellowships, I was fortunate (and as always very grateful) for the opportunity to go to their annual meeting and listen to talks from incoming and outgoing Fellows.  These meetings are tremendous - a very rare chance to hear 20 minute talks on topics across science, engineering, and math, aimed at the technically literate non-expert.  Back in the dim past of this blog, I've posted about these before (here, here, and here).  Here are some take-away facts I learned this time around:

• By very narrow targeting of specific pathogens, it might be possible to remove some of the evolutionary pressure (exerted by horizontal gene transfer [something I'd never learned about] from your gut bacteria) that leads to antibiotic resistant strains.
• It's possible to use ideas from superresolution microscopy and principal component analysis to improve structure determination in materials characterization.
• Using small molecule dyes, it is possible to use optical processes to turn the tables on some chemical reactions, favoring "anti-Markovnikov" selection, rather than Markovnikov rules (where reaction sites are determined by permanent dipole moments of bonds).
• Sometimes cells can recognize themselves (and distinguish between themselves and close relatives) using proteins based only on one or two genes.
• I'm used to thinking about coupling two (identical) resonators and getting an energy splitting (like bonding/antibonding orbitals).  I hadn't realized that using an effectively imaginary coupling means you can get a lifetime splitting (one long-lived, one short-lived mode).
• You can tie vortex rings in knots.  Watch the videos!
• Greenland has not been ice-free for at least 350,000 years, and radioactive dating based on dust captured in the ice makes it possible to untangle even faulted or folded ice cores.
• Monsoons are complicated, even if you model a completely water-covered idealized planet.
• Every time a pair of neutron stars collide, they produce about one Jupiter mass worth of Au, while a core-collapse supernova makes about one lunar mass worth of Au.  As a result, even though colliding neutron stars are rare, half of the gold out there came from them.  (In case you were wondering, in all of human history we have mined about 165,000 tons of Au.)

I've left out many others.  As always, very cool.

Ionic liquids and gating - how much is chemistry?

I've written before (here, here and here) about the use of ionic liquids in condensed matter physics investigations.  These remarkable liquid salts, with small organic molecules playing the roles of both positive and negative ions, can be used in electrochemical applications to generate extremely large surface charge densities near electrode interfaces.  Many experiments have been published in the last few years in which ionic liquids are meant to induce (via capacitive coupling) large densities of mobile charge carriers within interesting solids at the solid/ionic liquid interface. 

One concern in these experiments has been the role of surface chemistry.  While the molecular ions themselves are intended to be stable over a large range of electrochemical conditions, the ionic liquids can dissolve more reactive species (like water).  Likewise, recent experiments by Stuart Parkin of IBM Research have shown that in some systems (vanadium oxide in particular), under certain electrochemical conditions it would appear that ionic liquids can favor the formation of oxygen vacancies in the adjacent solid.  Since oxygen vacancy defects in many oxide materials can act as dopants, changing the concentration of charge carriers, one must be extremely careful that any measured changes in electronic properties are really from electrostatics rather than effective chemical doping. 

These concerns can only be ratcheted higher by the simultaneous online publication of two more papers from the Parkin lab, this one in Nano Letters (on SrTiO₃) and this one in ACS Nano (on TiO₂).  In both systems, the authors again find evidence that changes in oxygen stoichiometry (rather than pure electrostatic charging) can be extremely important in generating apparently metallic 2d surface layers.  

This is a very subtle issue, and the gating experiments remain of great interest.  Unraveling the physics and chemistry at work in all the relevant systems is going to be a big job, with a strong need for in situ characterization of buried solid-liquid interfaces.  Fun, challenging stuff that shows how tricky this area can be.

New big science prizes - time for nominations and opinions

There are large, endowed prizes in a number of disciplines.  The most famous of all are the Nobel prizes, of course, and in the sciences at least (chemistry, physics, and medicine), being awarded a Nobel is a singular crowning achievement.  A huge amount has been written about the Nobels - if you want to learn how they came to be, and at the same time become extremely disillusioned about the process for the early awards (my goodness I hope it's better these days - it seems like it must be), I recommend The Politics of Excellence.   The purpose of the Nobel is to reward a major, transformative (to use the NSF's favorite word) intellectual achievement.  The money is not meant to be a research grant.  (Similar in spirit is the Fields Medal for mathematics, though that is much less money and purposefully directed at younger researchers.)

The MacArthur Fellowships are another well-known set of awards.  These are known in popular parlance as "Genius Grants", and unlike the Nobels are (apparently) intended not so much as a financial reward, but as a liberating resource, a grant that can provide the winner with the financial freedom to continue to excel.  In some disciplines (the arts and the humanities in particular) this can completely change the financial landscape for the winners.   Awards that go directly toward furthering the creative ends of the recipients are clearly great things.

In recent years, a couple of new, very large awards have been created, and it's interesting to consider whether this is a good thing.  The Kavli Foundation is awarding prizes every other year in Neuroscience, Astrophysics, and Nanoscience.  To nominate someone, see here.   In spirit, these seem much like the Nobels, with awards so far going to extremely well regarded people, and not meant to function as direct research support.

In more flamboyant style, Yuri Milner has endowed the Fundamental Physics Prizes, also not meant to function as research grants.    What really distinguishes these latest, apart from the sheer magnitude of the awards ($3M each), is that they have largely gone to high energy physics theorists whose work has not been confirmed by experiment (in contrast to theoretical physics Nobel awards).  More recently there has been a special award to the LHC experimentalists, and some related prizes to condensed matter theorists.  However, the idea of giving very large prizes for unconfirmed theoretical work is controversial.  In essence, is something a "scientific breakthrough" if it's not confirmed by experiment, or is it very exciting math?  Perhaps this is just a labeling issue, but it is hard not to be unsettled by the willingness of some to try to detach science from experimental tests.

Is the scientific community better off from having more of these kinds of prizes?  Certainly it makes sense to consider awards for fields not recognized by the Nobel Foundation.  Nobels have gravitas because of their long established history, but that does not mean that there shouldn't be an analogous prize for, e.g., computer science.  Likewise, anything positive about the sciences that gets public attention is probably a net good.  However, prizes will lose their meaning if there are too many, and making some of them destabilizingly large amounts of money is not necessarily great.   It's also not clear quite what the point is if the same people win multiple large prizes for the same work.   For example, it's credible that Alan Guth could win a Nobel in addition and a Kavli astrophysics prize in addition to the Fundamental Physics prize.    I always tell would-be scientists not to get into this if they're after the big prize at the end - that's not the point of the enterprise, and I'd hate to see that change.  It's also hard for me to believe that the existence of these prizes is going to get the public or students materially more interested in the sciences.   Somehow prizes that go toward helping people continue their work or recognize a career of achievement seem more sound to me, but I remain ambivalent.

How to: Carry on a scientific collaboration

I'm writing this at the suggestion of a commenter on my previous how-to post, who was specifically interested in experiment/theory interactions.  Collaborations, as a fundamentally personal endeavor, are as varied as the people who collaborate.  Over the years I have collaborated with a number of theorist colleagues as well as fellow experimentalists, and generally it's been a very positive set of experiences, both scientifically as well as personally.  The main recommendations I can make about collaboration:

• Discuss and plan the ground rules at the beginning.  How is the collaboration going to work?  Is this the sort of collaboration that requires regular discussions and updates?  Are physical samples being sent by one party to another?  Which people are going to be responsible for what tasks?  What are peoples' expectations of authorship (recognizing that occasionally work may take an unanticipated turn, and someone's contribution may grow or shrink along the way)?  Are there restrictions about the samples or data?  (For example, a materials grower might collaborate with person A and person B on different projects; it could be very awkward if person A took samples and then on the side started working on the same project as person B!)
• Collaborate with people who have a similar approach to research projects as you, in terms of rigor, timeliness, and seriousness.  This is true whether those people are your own group, or outside collaborators. 
• Make sure to understand what your collaborators are actually doing.  Collaborations are a chance for you to learn something, since presumably you're working with these people because they bring something to a project that you can't do your self.  Sometimes asking what might seem at first glance a silly or naive question can lead to discussion that is informative for everyone.
• Have realistic expectations.  On the sociological level, realize that no one is going to retool their entire research enterprise or retask several people for your sake.  On the scientific side, know what can and can't be done by your collaborators and their techniques.
• Be communicative.  Keep your collaborators in the loop and up to date on what's going on.  If there is a big delay on your end for some reason, let them know.  You'd want them to do the same.  If you have decided that you don't think the project is going to work, or it's not working as anticipated, bring this up and don't let it sit.
• Be a finisher.  The most successful grad students are the ones who actually finish tasks and projects.  In the same way, don't let things slide.  If your collaborator wants you to read through a draft, or you promised to get some data to them in time for some deadline, follow through. 

Online access to papers + university libraries - info wanted

Now that we live in the Information Age, where I am reliably told that Information Wants to be Free, I'm confused by a trend that is coming in terms of how university researchers access electronic versions of journals.  (For those of you under 30, there was once a time when journal articles were published in an arcane format that predates pdf called "paper".)   The electronic availability of journals, including historical archives, has largely been an enormous boon to scientific progress.  It is far easier and faster now than ever before to do proper literature research when writing a paper or a proposal.  If I'm using google scholar or Web of Knowledge or Scopus or any other reference crawling aid, I can now find and (and if my institution subscribes or the content is available free) download copies of relevant references very quickly and efficiently.  If anything, the technology to provide this content is continually becoming cheaper and faster, since providing print content has far lower bandwidth requirements than the streaming video demands that are really driving innovation.

That is why I am concerned and confused by a trend popping up in the perpetually-financially-stressed university libraries around the country (and the world, presumably).  We all know that commercial publishers have been cranking up prices and applying annoying/evil tactics like bundling one high impact title with a dozen expensive, low-impact journals in forced package deals.  (Wiley, Elsevier, Taylor and Francis, that's you.)  Now, though, there is this idea being pushed that it would somehow be cheaper for university libraries to actually drop their subscriptions (!!) and instead use Get It Now, a product of the Copyright Clearance Center (those people you have to contact if you want permission to use a figure in a review article).  The problem is, Get It Now is misnamed; really it's Get It In Seven Minutes.  Needless to say, if you are trying to trace references and write a paper or proposal, having to wait seven minutes for every article you want to examine (which could easily number in the dozens while proposal writing) would be a major mess.   

Given that the publishers have the capability to provide content essentially instantly, and that the infrastructure to support that capability is steadily getting cheaper, and that the publishers could quite readily track download statistics (and could charge per download if they really wanted to), I don't understand how Get It Now is a positive step.  Surely if per-article billing was an economically viable approach, the publishers would do it themselves, right?  The publishes are going to recoup their costs somehow, passing them along to CCC, and CCC will pass those along to the universities, so it's hard for me to see how interposing a middleman like CCC can really do anything except slow down researchers and make money for CCC.  This idea seems to go directly against the trend of open access, public archives, etc. 

Do any of my readers work at institutions that use this service?   How does it work for you?  Is it as annoying as it sounds?  Does it actually enable your university to save money (that is, provide more or better content for the same actual cost) relative to the old approach?  A major challenge faced by universities in budgeting is that libraries don't sound as exciting as new buildings or major initiatives, and yet libraries and their services are essential to the scholarly mission of the institution.

How to: Write a response to referees

I think I'm going to start a periodic series of "how to" posts.  First up, how to write a decent "response to referees" document.  While this is pretty much common sense, it's not bad to think about it a bit in the abstract, rather than in the heat of the moment of having just received some kind of (perceived) searing blast of criticism.  In brief, assuming you get some collection of referee reports, at least one or two of which are not particularly positive, and you intend to revise and resubmit:

• Read the reports, and then put them aside for a day, as your white-hot rage over the terrible injustice that has befallen you fades, and in the cold light of reflection you realize that perhaps the manuscript you'd sent in is not, in fact, the greatest non-fiction prose writing since Churchill's six volume history of the Second World War.
• Now that you're in a less annoyed frame of mind, read through the reviews again, carefully, trying to understand (a) what the reviewers are actually saying, and (b) what the reviewers want you to do (assuming that's not "dry up and blow away").  Often the answers to (a) will reveal either that the reviewers did not properly understand the main point or some subsidiary point of the paper.  Much as we like to grumble about referees, you may have to admit that the fault could lie in your presentation.  Were your figures unclear?  Did the abstract and the intro make your main point explicit, or did you bury the lede somewhere down in the conclusions?  Remember, scientific papers are not mystery stories.  Springing the cool observation on the reader after a lot of setup risks the reader not realizing that the observation is cool.  Moreover, often the answers to (a) will reveal that the reviewer has thought of a possible concern or objection that you either didn't consider, or you did consider but dismissed without pointing it out and explaining your reasoning.  An extremely important part of the response process is figuring out what the main point of the referee is, and realizing that frequently it's worthy of consideration.
• Regarding (b) above, write down and make a list of what you think the referees want you to do, or what you think it would take to address the points that they raise.    Then consider whether you want to or should do all of those things.  Sometimes the referees can be very demanding.  (We've all seen this.)  You have to use your judgment, and remember that referees are not generally gratuitously mean.  I'd say the default position should be to do what they want, unless what they want is really considered unreasonable by you and your coauthors.  This list, by the way, is a headstart on the eventual "list of changes" that you'll need to provide when you resubmit.
• When you sit down to write your response, have the referee remarks right there.  In fact, it's a good idea to use copy/paste to intersperse your point-by-point responses.  That way you can be sure you didn't miss anything, and you are forced to write your response in an order that will seem logical to the referee.  
• Always (always) thank the referees for your time.  Seriously.  You know what refereeing is like, and you'd like to be thanked, admit it.  
• Point out that after this process you believe the paper is much improved (it will be, too, assuming the referees were really on point and not just asking you to cite their seminal work on the topic at hand), and if possible explain why.  (e.g., we believe that our main point is now much clearer)
• Always be polite and professional.  If you fly off the handle in your response, even if the referee is overtly hostile, it won't do you any favors with other referees or the editor.  Similarly, just as tone is difficult to convey in email, I suggest avoiding attempted jokes or sarcasm.  This is a professional communication - keep it that way.
• Try to be timely about revisions.  It's much better to get revisions done while everything is fresh in your mind, rather than letting things linger.  (Don't write them in the heat of the moment, though.)

In your accompanying cover letter when you resubmit, make sure that you emphasize the changes you made in response to the referees.  Also, it doesn't hurt to point out to the editor if you think a referee either missed the mark or seems not to be objective, but it would be best to do so in a very professional way.  Calling the referee an idiot won't win you any friends, particularly since the editor likely chose the referee.  Still, if you really think the referee made serious mistakes, or was not competent, or didn't read the paper, you should bring that to the editor's attention in a professional way.  Again, this kind of response is an important part of your repertoire of professional communications, so it's best to get in the habit of writing them well.

That's it for now.  I'm sure I've left out points - please feel free to bring them up in the comments.

Rankings and metrics - yet again

I (along with my departmental colleagues) was very happy to see this.  My department does extremely well in a particular ranking scheme (described in the original paper here and implemented online here, though you need to ask for password access) that asks, essentially, what fraction of the papers published by a department fall into the top 10% in terms of impact in an area.  We can debate about the flaws of any ranking scheme (hint:  they're all imperfect, because quantifying scientific quality and impact in a single number is fundamentally wrong-headed).  Still, it is nice to see an approach that agrees well with much intuition (that is, the usual top-10 suspects all look pretty good; schools that don't do much research in an area rank lower) where Rice does well. 

A new kind of solid - why "q-glass" really is weird and interesting

As long-time readers here know, I'm not a big fan of hype and press releases.  While it is very important to let people know what academic scientists and engineers are doing, not everything needs to be trumpeted from the rooftops as a huge breakthrough or a paradigm-altering thunderbolt.  However, this new result (the actual paper is here) is genuinely weird, unexpected, and exciting (at least to me).  The authors claim to have discovered a truly new kind of solid.  Let me break down what this means and why it's surprising.

A solid is a material that resists shear deformation - if you exert a certain force horizontally across the top surface of the material, the material will deform a bit until it's internal forces balance your applied force, and then deformation will reach some constant amount.  (In contrast, a fluid will keep deforming continuously!)  The most ordinary solids people know about are either crystalline (this includes polycrystalline materials made up of many crystal grains) or glasses.  In a crystalline solid, the atoms have taken on highly symmetric spatial arrangements.  That is, the atoms aren't separated by random distances, but integer multiples of certain particular spacings; similarly, crystals are not isotropic - there are particular directions along which atoms are arranged.   In contrast, simple liquids are isotropic, and except for some typical nearest-neighbor distance set by the atomic or molecular size, there is no other spatial ordered arrangement.   When a solid crystallizes from a liquid, it is a collective phenomenon, a phase transition, and this happens on cooling when the free energy of solid phase becomes lower than that of the liquid phase.  Quasicrystals (see 2011 Nobel for Chemistry) are in these senses crystals - their symmetries are just more subtle than those of ordinary crystals.

Glasses (including those made from polymers) are different.  They resist shear, too, but they do not have the long-range, periodic/anisotropic arrangement of constituents seen in crystals.  Instead, upon cooling, glasses become solid (meaning that their viscosity diverges toward infinity) because the constituents become "kinetically hindered".  At the risk of dragging up controversy, the simple description is that there is no true glass phase in the thermodynamic sense - glasses are rigid because the constituents can't readily move out of each others' ways, not because there is some true collective thermodynamic stability (involving free energies) at work. 

The authors of this new work have found something special that they have termed a "q-glass" while looking at what happens in the solidification of a molten mixture of aluminum, iron, and silicon.  In the resulting solids, they find nodules of a new material (Al₉₁Fe₇Si₂, approximately) that is definitely not crystalline or polycrystalline (no preferred lattice spacings; completely isotropic).  At the same time, the material does form out of the melt through a genuine first-order phase transition (!), and therefore appears to be highly ordered in some sense (both distinguishing it from a glass).  It will be very interesting to learn exactly what is going on here, and whether there are other materials that have these peculiar features.

Peer review, tone, and common courtesy

I'm back, though now I'm in the writing-six-things-before-the-term mode, so blogging will likely continue to be sparse.   Several of my friends pointed out this article in the Chronicle of Higher Education regarding the tone of correspondence in the peer review process.  In short, some fraction (from my own experience, I'd say maybe 15%) of "negative" reviews go beyond pointing out issues that need to be corrected to improve the paper and instead are genuinely hostile and nasty - basically tone and phrasing that the reviewer would very likely never have the nerve to use face to face.  Interestingly, those of us who experience the peer review process beat the rest of the world to the observation of a behavior pattern that is now realized to be common on the internet.

I wish I knew the solution to this.  Removing the blindness of the review process is one possibility, though I do worry that the same petty, vindictive people who write reviews like this will then engage in additional unprofessional behaviors toward people that they perceive as slighting them. 

The point of the review process in science is to make sure that correct, clear, original science results get disseminated in the literature.  We are all (allegedly) on the same side.  If people would just adhere to that, then reviews could be much more constructive in tone (e.g., instead of "The authors are just plain wrong", wouldn't it be better to say "I'm concerned that there are some problems with steps 1 through 4"?).

I am worried that there is a general erosion in common courtesy as well.  I know this makes me sound like a grumpy old man, but again there are some people who use electronic communications in general as an excuse for rudeness.  Taking the time to say "please" and "thank you" is never time poorly spent.

Online physics lecture notes

Next week is going to be largely internet-free for me, so don't expect much excitement here until early August.  I do have a few things I want to discuss (thermoelectric effects, the great mess that was San Jose State's experiment in MOOCs, etc.), but that will wait.  In the mean time, I wanted to point out some nice lecture notes I'd found lately on the arxiv and elsewhere.  If people have their own favorite notes to point out online, please do so in the comments.

Neri Merhav has produced a couple of nice sets of notes, written from the perspective of trying to teach very physicsy concepts to electrical engineering students.  This past week he put up these notes about statistical mechanics, and previously he had written this set about the connections between information theory and statistical physics.  I found them both very readable.

Doron Cohen's notes on statistical mechanics and mesoscopics are a bit more mathy and closer to notes than a textbook-style discourse.

Not on the arxiv, but Yoshi Yamamoto's online notes regarding noise and noise processes are great.

Printing at the 180 nm scale??

I stumbled across this post, where it is asserted (with no link, and my google-fu is inadequate) that some group or collaboration at Berkeley is working on the ability to make 180 nm critical dimension transistors via printing (and therefore over really large areas, rather than only on dinner-plate-sized Si wafers).  Can a reader out there point me to who is really doing this, or is this a case of a press office distorting things (e.g., taking a layer thickness and claiming it's a lateral feature size)?

Physics is hard - how much should that worry us?

I'm a bit late to the party, but there have been discussions lately about the number of undergrad STEM majors, including physics, with some gnashing of teeth about overall difficulty. For example, a report by the National Bureau of Economic Research has been interpreted as saying that the main reason students bail on science majors is poor grades. That is, students go in, knowing that science will require more work than majoring in something fluffy, but when many receive tough grades even though they work hard, that's too much for them and they change fields. Chad Orzel does his usual thorough job looking into what the study really says, and it does seem true that tough grading drives some people out of STEM pursuits.

Similarly, there is a new report from the National Academy of Sciences called "Adapting to a Changing World: Challenges and Opportunities in Undergraduate Physics Education". I found the content rather disappointing, in the sense that it didn't seem to say much new. We all know that some approaches can be better under some circumstances than traditional lecture. However, many of those are very labor intensive, and I'm sure that my 50 person class would benefit if it were instead five ten-person classes. More to the point, though, the report specifically claims that hard grades are a major factor in the low participation of women and underrepresented groups in the physics major.

So, is physics unnaturally harsh in its grading, to its detriment? Or is this a question of high school preparation on the one hand, and grade inflation in nonscience majors on the other? I lean toward the latter.

(Note that the NSF has proven that science is hard. Also, here is the paper featured in that article - it's actually very interesting.)

(One other note: no one commented on my three part post about the physics of contacts, and the hit rate on those posts was very low. At the same time, in one 15 minute interval last week my post about "whiskey stones" got nearly 500 page views after it was mentioned in an argument about whiskey on reddit. Guess I should write about other things besides physics if I want more readership:-).

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Contacts III: The search for measurements

In the last two posts I've talked a bit about contact resistances, but I haven't said much of anything about how to infer these experimentally. 

In some sense, the best, most general way to understand contact voltages is through scanning potentiometry.  For example, this paper (pdf - sorry for the long URL) in Fig. 10 uses a conductive AFM tip to look at the local electrostatic potential as a function of position along an organic transistor under bias.  When done properly, this allows the direct measurement of the potential difference between, e.g., the source electrode and the adjacent channel material.  If you know the potential difference and the current flowing, you can calculate the contact resistance.  Even better, this method lets you determine the \( I-V \) characteristic of the contact even if it is non-Ohmic, because you directly measure \(V\) while knowing \(I\).   The downside, of course, is that not every device (particularly really small ones) has a geometry amenable to this kind of scanned probe characterization.

A more common approach used by many is the transmission line method.  In the traditional version of this, you have a whole series of (otherwise identical) devices of differing channel lengths.  You can then plot the resistance of the device as a function of \(L\).  For Ohmic contacts and an Ohmic device, the slope of the \(R-L\) plot gives the channel resistance per unit length, while the intercept at \(L \rightarrow 0\) is the total contact contribution.  This does not tell you how the contact resistance is apportioned between source/channel and channel/drain interfaces (this can be nontrivial - see the figure I mentioned above, where most of the voltage is dropped at the injecting contact, and a smaller fraction is dropped at the collecting contact).  Related to the transmission line approach is the comparison between two- and four-terminal measurements of the same device.   The four-terminal measurement, assuming that no current flows in the voltage contacts and that the voltage probes are ideal, should tell you the contribution of the channel.  Comparison with the two-terminal resistance measurement should then let you get some total contact resistance.  I should also note that, if you know that the channel is Ohmic and that one contact dominates the resistance, you can still use length scaling to infer the \( I-V \) characteristic of the contact even if it is non-Ohmic.

The length scaling argument to infer contact resistances has also been used to great effect in molecular junctions.  There, for non-resonant transport, the usual assumption is that the bulk of the molecule (whatever that means) acts as an effective tunneling barrier, so that conductance should fall exponentially with increasing molecular length (assuming the barrier height does not change with molecular length, an approximation most likely to be true in saturated as opposed to conjugated molecules).  Thus, one can plot \(log G\) as a function of molecular length, and expect a straight line, with an intercept that tells you something about the contact between the molecule and the metal electrodes.  This has been done in molecular layers (see here, for example), and in single molecule junctions (see here, for example).  These kinds of contact resistances can then be related, ideally, to realistic electronic structure calculations looking at overlap between electronic states in the metal and those of the linking group of the molecule.

Hopefully these three posts have clarified a little the issue of contact effects in electronic devices - why they are not trivial to characterize, and how they may actually tell you interesting things.

Contacts, part deux

I will make an argument now that contact resistances are much maligned, and instead of rigorously trying to avoid worrying about them, we should instead look for opportunities (with well defined, reproducible contact interfaces) when they can actually tell us something. I'll punctuate this with some papers from our own group and areas I happen to know, but that's only because those are the examples that come to my mind.

What happens when you try to inject charge from a metal into a hopping conductor - a material with some energy-dependent density of localized states? Many organic semiconducting polymers are such systems. In this situation, an injected charge carrier faces a competition between diffusion away into the channel by hopping, and an attraction to its own image charge in the metal. The rather odd result is that this contact often tends to be Ohmic (in the sense that the contact voltage is directly proportional to the current), but the contact resistance ends up being inversely proportional to the mobility of the charge in the channel. This is true even when the metal Fermi level lies somewhere in the tail of the band (a situation where you would expect a Schottky contact in a nonhopping semiconductor). We ran into this here, and systematically varied the contact resistance by using surface chemistry to adjust the energetic alignment.

In correlated materials, the situation may seem tantalizing yet hopeless. On the one hand, you know something interesting must happen when charge is injected into the material - carriers in the metal are boring, electron-like quasiparticles, while charge excitations in the correlated system could in principle be very different, with fractional charge or spin-charge separation. On the other hand, depending on the bulk properties and ability to make reproducible contacts, it can be very hard to extract useful information from contact resistances in these systems. We did get lucky, and found that in magnetite conduction in both the high temperature (short range ordered) state and in the low temperature (long range ordered) state seems to be through hopping, similar to the description above. I definitely think that there is a lot more to be done in such materials by using contact effects as a tool rather than avoiding them.

In the world of molecular junctions, often one is in the limit where the device is "all contact", in the sense that the "bulk" is only a couple of nanometers and a few atoms. Next time I'll talk about some great measurements by others in these systems, as part of a discussion on how one can measure contact resistance.

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Contacts - annoying or an opportunity

In condensed matter physics, often we are interested in the flow of charge from a "source" electrode, through some material (the "channel", probably a different material than the source), and away into a "drain" electrode.  When the source, channel, and drain are all metals, life is simple.  While there might be some mismatch in the electrical conductivities of the materials, in the end an electron can go from some delocalized (extended, wavelike) state in the source, into such a state in the channel, and then into such a state in the drain, smoothly.   Because of the discontinuity in electronic band structure and dielectric properties, there is some reflection at the interface, leading to a contact resistance.  That is, some fraction of the applied voltage, linearly proportional to the applied voltage, is dropped across the source-channel contact, and some across the channel-drain contract.  This is an example of an ohmic contact.

However, the situation can be more complicated.  If the channel is a crystalline semiconductor, the Fermi level of the metal usually winds up sitting somewhere in the band gap.  If the is appropriate band bending takes place, there can then be an energy barrier (a Schottky barrier) for injection if charge from the metal into the semiconductor.   The spatial width of the barrier depends on the level of doping in the semiconductor, with higher doping leading to a narrower (though not necessarily shorter) barrier.   In this case, the current-voltage characteristics of the contact is not Ohmic, and looks instead like a diode, because the applied bias changes the shape of the barrier.  To avoid this in transistors, the regions of the channel where the source and drain contact it are very highly doped.  Still, in this case we are still assuming that the actual electronic states are extended, delocalized things.

The situation gets more complicated when the channel does not have delocalized states near the Fermi level.  

Usually experiments are designed to mitigate contact effects, either by avoiding measurements of the contact voltages (so-called four terminal measurements) or by making the contact contribution negligible compared to the bulk channel.  However, it turns out that sometimes contact effects can provide valuable insights into charge transport properties in the bulk.  I'll write more soon about this.

Timescales, averaging, and baseball

Please pardon the summer blogging slowdown - it's been a surprisingly busy couple of weeks, between an instructor search, working on papers, proposal stuff, and trying to write more on my big long-term project. 

Thanks to an old friend for pointing me to this link, which does a great job looking at why a knuckleball is so erratic in its flight from pitcher to batter.  For non-Americans:  In baseball, a pitcher throws a ball to a catcher, while a batter attempts to hit the ball.  There are several types of pitches, depending on the pitcher's grip on the ball (which has seams due to the stitching that holds the leather cover on), the throwing motion, and the release.  A fastball can reach speeds in excess of 100 mph (161 kph) and typically spins more than 1000 rpm.  In contrast, a knuckleball can drift by the batter at a leisurely 70 mph yet be nearly unhittable because of its erratic motion.   A knuckleball barely spins, so that it may complete only 1-2 revolutions from leaving the pitcher's hand to reaching the batter.  This means that the positioning of the seams is absolutely critical to determing the aerodynamics of the motion, and no two knuckleballs move the same way.  In physics lingo, a knuckleball has almost none of the orientational averaging that happens in basically every other pitch.  I propose the definition of a new dimensionless parameter, the Wakefield number, \(W\), that is the ratio of the ball's period of revolution to its time-of-flight from pitcher to batter.   A knuckleball is a pitch with \(W \sim 1\).

Come on, PRL editors.

I rarely criticize papers.  I write this not to single out the authors (none of whom I know), nor to criticize the actual science (which seems very interesting) but to ask pointedly:  How did the editors of PRL, a journal that allegedly prizes readability by a general physics audience, allow this to go through in its current form?  This paper is titled "Poor Man’s Understanding of Kinks Originating from Strong Electronic Correlations".  A natural question would be, "Kinks in what?".  Unfortunately, the abstract doesn't say.  Worse, it refers to "the central peak".  Again, a peak in what?!   Something as a function of something, that's for sure. 

Come on, editors - if you are going to let articles be knocked from PRL contention because they're "more suitable for a specialized journal", that obligates you to make sure that the papers you do print at least have titles and abstracts that are accessible.  I'm even a specialist in the field and I wasn't sure what the authors were talking about (some spectral density function?) based on the title and abstract.

The authors actually do a good job explaining the issue in the very first sentence of the paper:  "Kinks in the energy vs. momentum dispersion relation indicate deviations from a quasiparticle renormalization of the noninteracting system."   That should have been the first sentence in the abstract.  In a noninteracting system, the relationship between energy and momentum of particles is smooth.  For example, for a free electron, \( E = p^{2}/2m \) where \(m\) is the mass.  In an ordinary metal (where Fermi liquid theory works), you can write a similar smooth relationship for the energy vs. momentum relationship of the quasiparticles. Kinks in that relationship, as the authors say, "provide valuable information of many-body effects".